This section introduces aspects that may be helpful in facilitating a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admission about what is in the prior art.
Future radio communication networks may have a mixture of conventional first base stations being adapted to transmit radiation beams to several so-called horizontal directions but to only one so-called vertical direction and further more sophisticated second base stations being adapted to transmit so-called horizontal radiation beams to several horizontal directions and further being adapted to transmit so-called vertical radiation beams to several vertical directions. Horizontal direction means in the following a direction, which may be defined by an azimuth angle between a surface normal of a radiating surface of an antenna system and a beam direction of a horizontal beam. Vertical direction means in the following a direction, which is defined by an elevation angle between the surface normal of the radiating surface of the antenna system and a beam direction of a vertical beam.
When such a second base station transmits time multiplexed common pilots a mobile station which arrives in a coverage area of the second base station would observe only an aggregated vertical radiation beam by measuring the different pilots over a certain period of time and would be not aware of the several vertical radiation beams. Thus, the mobile station or a base station currently serving the mobile station may select a target radio cell of a further second base station for a handover, although one of the several vertical radiation beams of the second base station is directly pointing to the mobile station. Furthermore, the serving base station might not know an impact of interference caused by the several vertical radiation beams at various locations in its own serving radio cell.
In current 3GPP standards (3GPP=3rd Generation Partnership Project), only two radiation beams with a first and a second measurement area can be supported based on time domain RS and CQI measurements (RS=Reference Symbols, CQI=Channel Quality Identifier) which have been originally introduced for eICIC (eICIC=enhanced Inter Cell Interference Coordination). Instead of using the two measurement areas for a so-called ABS (ABS=Almost Blank Subframe) and a so-called non-ABS, the two measurement areas may be used for upper beam and lower beam. However, in current 3GPP standards such two measurement areas are only supported for CQI measurements inside a current serving cell and not for a handover to a target cell. Furthermore, not more than two radiation beams can be supported by such a technical consideration.
The best existing solution in case of having more than two vertical radiation beams at a target base station would be to measure average signal strength over all the vertical radiation beams which may lead to a non-optimal handover decision and a non-optimal radio cell selection and cell association. Another solution would be to define every of the vertical radiation beams as a separate radio cell. However, this would lead to many handovers and to a non optimal throughput performance, because a time consuming handover would be required in order to use the optimal beam and a fast switching between the radiation beams would be not possible.
US 2009/0264118 A1 discloses a method of generating neighbor lists in a radio communication network comprising user terminals and base stations defining sectored or omnidirectional radio cells. The method comprises the steps of receiving radio channel measurements from at least some of the user terminals, creating a first table which comprises for at least some of the user terminals the corresponding radio channel measurement in relation to different radio cells, processing the first table to generate a cell coupling matrix comprising coupling figures between different pairs of cells and obtaining for a certain cell a neighbor list which contains neighbor that have a coupling figure greater than a given threshold.